The present invention relates to reflective materials and treatments for enhancing solar reflectivity for use on exterior surfaces such as on asphalt shingle roofs, roofing tiles, and other exterior surfaces and methods to reduce energy consumption costs for cooling buildings.
For energy conservation purposes, it has become more desirable to reflect solar energy from roofs and other exterior surfaces. Absorbed solar energy increases cooling energy costs in buildings. In addition, in densely populated areas, such as metropolitan areas, the absorption of solar energy increases ambient air temperatures. A primary absorber of solar energy is building roofs. It is not uncommon for ambient air temperature in metropolitan areas to be at least 10 degrees Fahrenheit warmer than in surrounding rural areas. This phenomenon is commonly referred to as the urban heat island effect. Reflecting solar energy rather than absorbing it can reduce cooling costs and thereby energy costs in buildings. In addition, reducing solar energy absorption can enhance the quality of life in densely populated areas by helping to decrease ambient air temperatures.
Reflection of solar energy can be accomplished by using white or light-colored roofs. However, such light-colored roofs are not well accepted in the marketplace for aesthetic reasons. Instead, darker roofs are preferred. However, darker roofs by their very nature absorb a higher degree of solar energy and reflect less. To diminish the absorption of solar energy without affecting the visible color of the roof, enhanced reflection in the infrared portion of the spectrum is desirable.
Non-flat or sloped roofs typically use shingles coated with colored granules adhered to the outer surface of the shingles. Such shingles are typically made of an asphalt base with the granules embedded in the asphalt. The roofing granules are used both for aesthetic reasons and to protect the underlying base of the shingle. The very nature of such granules creates significant surface roughness on the shingle. Solar radiation thereby encounters decreased reflectivity since the radiation is scattered in a multi-scattering manner that leads to increased absorption when compared to the same coating placed on a smooth surface.
Although construction materials may have sufficiently high solar energy reflectivity when they are installed, a variety of environmental factors tend to degrade that performance. Growth of microorganisms, such as algae, lichen, and moss, is a common problem on roofs in many areas especially those where exposed surfaces are often damp. In other regions, the deposit of air borne materials such as soot is a primary contributor to reduced solar energy reflectivity. These problems have been addressed as a nuisance appearance issue by the art.
The conflict between the aesthetic desire for darker construction surfaces and the energy efficiency which can be obtained through the higher solar energy rejection of white or near white surfaces has required a compromise that favors lighter colors. Although lighter colors may meet initial solar reflectivity standards such as that required for Energy Star® labeling, they tend to lose their reflectivity over time as dirt and microorganisms accumulate. Maintaining a useful or desired level of solar reflectivity for several years generally means that the initial reflectivity must be significantly higher than the eventual target and this, in turn, requires an even lighter, less desirable initial color.
In the case of the Energy Star® labeling standard, this decline in reflectivity has been recognized by the inclusion of an exposure aging requirement. The initial solar reflectivity for steep slope roofs must be equal to or greater than 25% and must remain greater than or equal to 15% after 3 years. It is desirable to maintain an even higher reflectivity. In regions having high pollution and/or moist conditions favorable for algal growth, it may be necessary to select materials with an even higher initial reflectivity, in some instances as high as 30%, in order to retain 15% reflectivity after 3 years.